1. Field of the Invention
The invention relates to an overvoltage protection element for discharging transient overvoltages, with at least two electrodes, with at least one ignition element of insulating material located between the electrodes, and with an air breakdown spark gap which acts between the electrodes, when the air breakdown spark gap between the two electrodes is ignited, an arc being formed. In addition, the invention relates to an ignition element for use in an overvoltage protection element.
2. Description of Related Art
Electrical, but especially electronic measurement, control and switching circuits, mainly also telecommunications equipment and systems, are sensitive to transient overvoltages, as can occur especially by atmospheric discharges, but also by switching operations or short circuits in power supply circuits. This sensitivity has increased to the extent electronic components, especially transistors and thyristors, are being used; in particular, increasingly used integrated circuits are highly endangered by transient overvoltages. Overvoltages can to a major extent destroy electrical and electronic equipment and systems. Damage is not limited to industrial and commercial systems. Building hardware, including household appliances, such as cooking appliances, telephone systems, television sets, hi-fi systems and computers, are affected. Without effective protective measures against overvoltages, high costs for repair or repurchase of the affected systems and devices must be expected.
Electrical circuits normally work without problems using the voltage specified for them, the rated voltage. This does not apply when overvoltages occur. Overvoltages are all voltages which are above the upper tolerance limit of the rated voltage. They also include mainly transient overvoltages which can occur due to atmosphere discharges, but also due to switching operations or short circuits in power supply circuits, and can be galvanically, inductively or capacitively coupled into electronic circuits. In order to protect electronic measurement, control and switching circuits, mainly also telecommunications equipment and systems, wherever they are used, against transient overvoltages, an overvoltage protection element and overvoltage protection means have been developed and have been known for more than 20 years.
An important component of overvoltage protection elements of the type under consideration here is at least one spark gap which operates at a certain overvoltage, the sparkover voltage, and thus, prevents overvoltages which are greater than the sparkover voltage of the spark gap from occurring in the circuit which is being protected by the overvoltage protection means.
It was stated initially that the overvoltage protection element of the type under consideration has two electrodes and an air breakdown spark gap which is located and which works between the two electrodes. An air breakdown spark gap is generally a breakdown spark gap; therefore, it is also to encompass a breakdown spark gap in which not air, but another gas is present between the electrodes. In addition to overvoltage protection elements with an air breakdown spark gap, there are overvoltage protection elements with an air flashover spark gap in which a creeping discharge occurs upon operation.
Overvoltage protection elements with an air breakdown spark gap compared to overvoltage protection elements with an air flashover spark gap have the advantage of a higher peak current carrying capacity, but the disadvantage of a higher sparkover voltage which is not especially constant. Thereforem various overvoltage protection elements with an air breakdown spark gap have been proposed which have been improved with reference to the sparkover voltage. In the area of the electrodes and the air breakdown spark gap which acts between the electrodes, ignition aids have been implemented in various ways, for example, such that at least one ignition aid which triggers a creeping discharge has been provided between the electrodes and projects at least partially into the air breakdown spark gap. Such an ignition aid is made in the manner of a bridge, and is made of plastic (see, for example, German Patent Applications DE 41 41 681 A1, DE 42 44 051 A1 or DE 44 02 615 A1).
The ignition aids which were addressed above and which are provided in the known overvoltage protection elements can, more or less, be called “passive ignition aids” because they do not operate “actively” themselves, but only operate by an overvoltage which occurs on the main electrodes.
German Patent Application 198 03 636 and corresponding U.S. Pat. No. 6,111,740 disclose an overvoltage protection element and an overvoltage protection means with two electrodes, with an air breakdown spark gap which acts between the electrodes, and an ignition aid. In this known overvoltage protection means the ignition aid is made as an “active ignition aid,” specifically in that, in addition to the two electrodes—there called the main electrodes—there are two more ignition electrodes. These two ignition electrodes form a second air breakdown spark gap which is used as an ignition spark gap. In this known overvoltage protection means, the ignition aid includes not only the ignition spark gap, but also an ignition circuit with an ignition switching element. When there is an overvoltage on the known overvoltage protection means, the ignition circuit with the ignition switching element provides for operation of the ignition spark gap. The ignition spark gap and the two ignition electrodes are arranged with respect to the two main electrodes such that, because the ignition spark gap has operated, the air breakdown spark gap between the two main electrodes operates. The operation of the ignition spark gap leads to ionization of the air present in the air breakdown spark gap so that the air breakdown spark gap between the two main electrodes also operates suddenly after the ignition spark gap operates.
In the known embodiments of the overvoltage protection elements described above with ignition aids, the latter lead to an improved, specifically a lower and more constant sparkover voltage. However, the disadvantage of an active ignition aid is that an additional ignition circuit with an ignition switching element is necessary to effect operation of the ignition spark gap. Here, there is the danger that the ignition spark gap and the ignition circuit with the ignition switching element will be destroyed by a lightning stroke current or the main follow current which generally occurs.